Based on winning a Gift Certificate from a department store, you can almost treat this like a 'earnings' although it's not really. We set aside the same % in taxes we get taken from our paychecks and that covered the hit on our taxes the following year just fine.
As long as you are taking it into consideration, you should be fine. I think you can even ask your employer to take out 'extra' from your checks to cover this in taxes.